1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as TFT) and to a semiconductor device having a circuit structured with a thin film transistor. The present invention relates to such semiconductor devices as electro-optical devices, typically active matrix liquid crystal display devices (hereinafter referred to as AM-LCDs), and semiconductor circuits including processors, etc. The present invention also relates to electronic equipment loaded with the electro-optical devices or semiconductor circuits. Note that throughout this specification semiconductor device indicates general devices that acquire their function through the use of semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic equipment are semiconductor devices.
2. Description of the Related Art
Active matrix type liquid crystal display devices composed of TFT circuits that use polysilicon films have been in the spotlight in recent years. They are the backbone for realizing high definition image displays, in which a multiple number of pixels are arranged in a matrix state, and the electric fields that occur in liquid crystals are controlled in that matrix state.
With this type of active matrix type liquid crystal display device, as the resolution becomes high definition in XGA and SXGA, the number of pixels alone exceeds one million. A driver circuit that drives all of the pixels is therefore extremely complex, and furthermore is formed from a large number of TFTs.
The required specifications for actual liquid crystal display device (also called liquid crystal panels) are strict, and in order for all of the pixels to operate normally, high reliability must be secured for both the pixels and the driver circuit. If an abnormality occurs in the driver circuit, especially, this invites a fault called a line defect in which one column (or one row) of pixels turns off completely.
However, from a reliability point of view, TFTs that use polysilicon films still fall behind MOSFETs (transistors formed on a single crystal semiconductor substrate), etc., used in LSIs. As long as this shortcoming is not overcome, the point of view that it is difficult to use TFTs when forming an LSI circuit will get stronger.
The applicant of the present invention considers that when comparing a TFT with a MOSFET, the problems associated with the TFT structure affect its reliability (especially hot carrier resistance).